It is known to use a reciprocating piston pump to pump liquid paint around a circuit which includes a storage reservoir and one or more take-off points serving one or more paint spraying guns. Reciprocating pumps are generally preferred to rotary pumps since they are much less likely to damage the pigments and other inclusions in the liquid paint.
It is known to drive a reciprocating pump by means of fluid pressure using pneumatic or hydraulic motors. However, such motors are relatively wasteful of energy, and attempts have been made to replace fluid motors by electric motors in order to save energy, and thus minimise running costs.
A problem of reciprocating pumps is the loss of pressure at the end of the pumping stroke, when the piston of the pump undergoes stroke reversal. Even in a double-acting pump, where both forward and return strokes of the piston are pumping strokes, there is nevertheless a significant drop in supply pressure at both ends of the piston stroke. In order to minimise this problem, and also to achieve a fast response in changing the pump cycle rate when responding to changes in paint pressure in the circulation system, it is necessary to use a servo motor as the electric drive motor. The servo motor, together with its control mechanisms can achieve rapid reversal of stroke, at the ends of the pumping stroke to minimise paint pressure “fall-off” and can also respond quickly to make changes in the pump cycle rate to maintain a predetermined pressure in the paint circulating system. However, the use of a servo motor has proved to be extremely expensive. Servo motors themselves are expensive items, and require expensive ancillary control equipment including digital encoders to provide an indication, at any time instant, of the position of the piston within its stroke, a relatively complex servo control arrangement utilising specialist computer software, a complex electrical installation, and will need a high level of electrical expertise to maintain the system. Thus a servo motor driven pumping system involves a high investment cost and has proved unattractive to prospective customers notwithstanding the fact that when in use such a system could effect energy savings by comparison with conventional fluid driven motor systems.
It is an objective of the present invention to provide a system in which the aforementioned disadvantages are minimised.